If you're new to doubles and not tech diving I'd suggest the 80's. Keeping within your NDL's you'll have more gas than you'd ever need. Keep the 100's apart so you have a bit of extra gas for single tank dives.

I really like the steel LP 85s. They fit nicely and trim out great. No need for any weight belt. When you you want to go deep and long you can overfill to around 3600 and have all the gas you need. I love ending a 200' deco dive with 2000 psi still on your back.

I wish I didn't still need a weight belt with my twinsets, but at least it's a lighter belt. I like the LP tanks (72s or 85s) for twins because they are easy to top off with a transfill whip. Also a little easier on the back than my Faber FX100s.